Originally Posted by
sysrq
With cup and cone you never know when they are perfectly adjusted. They are usually perfect at the beginning after overhaul but then after 100km they are loose again even with sufficient locknut adjustment while trying to avoid stripping the threads even with Deore level of precision. If you leave them tighter intentionally then they sometimes continue to self tighten.
Could it be that MKS is actually avoiding any interference fit on those two inboard bearings since otherwise it's impossible to share the load equally between them?
I had these problems when I was not adjusting my cone-and-cup bearings properly. Leaving the slightest bit of slack, concentrated radial loading in fewer balls at one time and they wore more. Not loosening in 100 km, but perhaps after a couple thousand. In more recent years, adjusting with exactly the right preload, and they stay great for years and tens of thousands of miles, they usually only need to get rebuilt if I get caught in a big rain and water and grit make it past the well-worn seals. As I detailed previously, no slack, just enough preload to feel all the bearing balls engaged (hard to describe but you'll know it when you feel it, spinning the spindle with your fingers), but not so tight it feels notchy. Perfect preload happens by creeping up on it very slowly when adjusting the bearings (and needs to be with locknut engaged as the threads have backlash, axial gap between male and female threads), and if it feels notchy, backing off slightly until it doesn't. Hard to describe, but the first time you feel a perfectly preloaded bearing, you'll never forget it.
Again, as FBinNY has said, the MKS design is not like cup-and-cone, so the above is not recommended, in that case follow manufacturer's instructions; I haven't seen it, but it's very easy for me to visualize a design where one bearing manages axial play in addition to taking half the radial load, and the other bearing handles just radial load. This is similar to a "full-floating" axle design on trucks, which allows the axle driveshaft to expand and contract axially without affecting the bearing adjustment. It's a smarter design.